A driver circuit for driving an optical modulator needs to amplify an electrical signal with a small amplitude (for example, about several hundred mV) to a large amplitude (for example, about several V) with which the optical modulator can be driven, and output the electrical signal. This driver circuit desirably has both a large gain characteristic and a wide-band characteristic, and is generally implemented by a group of multistage amplifiers that are connected.
FIG. 29 shows the main part of a related driver circuit. A driver circuit 300 gradually amplifies the amplitude of an input signal Vin using preamplifiers 11-1 and 11-2 and an output-stage amplifier 12, and outputs an output signal Vout having a desired amplitude. This eliminates the need to increase a gain for an amplifier of each stage, thereby implementing a wide band characteristic in each amplifier. Therefore, it is possible to obtain a driver circuit, as a whole, having both a large gain characteristic and a wide band characteristic.
In general, as a necessary output amplitude increases, a power supply voltage necessary for an amplifier increases as well. Thus, a power supply voltage that is necessary to drive each amplifier becomes higher in a later-stage amplifier. In the driver circuit 300 shown in FIG. 29, positive-side power supply terminals 11-1a and 11-2a of the preamplifiers 11-1 and 11-2 are connected to a supply line (positive-side power supply line) LA of a power supply voltage V1, and negative-side power supply terminals 11-1b and 11-2b of the preamplifiers 11-1 and 11-2 are connected to a ground line (negative-side power supply line) GND. In addition, a positive-side power supply terminal 12a of the output-stage amplifier 12 is connected to a supply line (positive-side power supply line) LB of a power supply voltage V2 (V1<V2), and a negative-side power supply terminal 12b of the output-stage amplifier 12 is connected to the ground line (negative-side power supply line) GND. With this, the power supply voltage V1 is supplied between the positive-side power supply terminals 11-1a and 11-2a and negative-side power supply terminals 11-1b and 11-2b of the preceding-side amplifiers 11-1 and 11-2, and the power supply voltage V2 is supplied between the positive-side power supply terminal 12a and negative-side power supply terminal 12b of the output-stage amplifier 12.
Note that non-patent literature 1 describes a circuit arrangement of decreasing the power consumption of a driver circuit by driving, with a power supply voltage lower than that of an output-stage amplifier, an amplifier provided in the preceding stage where a relatively small amplitude is processed.
In the driver circuit 300 shown in FIG. 29, assuming that if all amplifiers (the preamplifiers 11-1 and 11-2 and the output-stage amplifier 12) are driven by the power supply voltage V2, the current consumption of each amplifier remains unchanged, the power consumption is given by V2×(2I1+I2). However, if the preamplifiers 11-1 and 11-2 are driven by the power supply voltage V1, it is possible to reduce the power consumption by (V2−V1)×2I1.